Coated papers or cardboards known in the art usually exhibit at least one coating layer containing pigments and binders. The most commonly used pigment is a clay such as kaolin, which is often associated with other pigments such as calcium carbonate, titanium oxide, hydrargillite, talc or barium sulfate. These pigments are usually fixed to the paper or cardboard with one or a mixture of the following binders: a styrene butadiene copolymer, an acrylic polymer, a vinyl acetate polymer, natural binders, starch, proteins, caseins, or polyvinyl alcohols.
The coating layer may also include dispersing agents such as tetrasodium pyrophosphate, sodium hexametaphosphate or low molecular weight polyacrylates, present in proportions of between 0.2 and 0.5% by weight in relation to the pigments. Agents such as carboxymethylcellulose and alginates, which are used to improve water retention properties, may also be present in the coating layer.
The requisite properties of a given coated paper or cardboard differ according to the printing method which is to be used: rotogravure, offset, letterpress, flexograph, silkscreening, etc. In the rotogravure process, ink is contained in small wells and its transfer by capillarity can occur only if the periphery of each well is in precise contact with the coated paper or cardboard onto which it is to be transferred. Because of this, a high smoothness coated paper or cardboard must be used. The higher the calendering pressure, the higher the smoothness obtained. However, in order to maintain a layer porosity sufficient for insuring the printing quality of the coated paper or, in the case of cardboard, to maintain physical or mechanical properties such as thickness and/or rigidity, the calendering pressure must be limited. Alternatively, a heavy-weight paper or cardboard--10 to 20% greater in weight--must be used. Due to these limitations, coated papers or cardboards known in the art, such as those coated with compounds containing mineral pigments, are generally not suitable for rotogravure printing.